In recent years there has been an explosion of various wireless technologies such as WiFi (802.11), 3G (CDMA), WiMax (802.16) and many others. All of these technologies use different modulation schemes and access methods, such as code division multiple access (CDMA), orthogonal frequency division multiple access (OFDMA), or the like. To access network services using these different technologies, multi-mode mobile devices have been developed. Multi-mode devices are end-user devices configured to selectively interface to different wireless technologies. They allow users to connect to wireless communications networks irrespective of the underlying access technology.
Users of multi-mode wireless devices are mostly interested in network support for their particular applications, regardless of the underlying access technology. More importantly, users are interested in receiving uninterrupted services regardless of the access technology. Uninterrupted service is possible through a system design concept called seamless mobility. Seamless mobility provides the user with seamless access and connectivity across different wireless technologies and different wireless networks, such as wireless local area networks (WLANs), including WiFi networks, and wireless wide area networks (WWANs), such as cellular phone networks. Handoffs from WLAN to WWAN technologies should be transparent to the user, allowing a simplified, seamless mobile communications experience.
Although WWAN air interface technologies such as GSM, CDMA2000 and the like are designed to cover region surrounding their base stations (cells), not all areas within the intended coverage regions can receive WWAN service. These areas that lack coverage are typically known as coverage holes. WWAN coverage holes frequently occur inside buildings. To obtain wireless services within buildings or other coverage holes, multi-mode devices can often connect to a WLAN providing coverage within the building or WWAN coverage hole. However, in making a WLAN/WWAN hand-off, multi-mode devices operating in some indoor environments experience dropouts, which interrupt their service. To provide true seamless mobility, multi-mode devices operating indoors or in other types of WWAN coverage holes should be able to transparently acquire uninterrupted WWAN services when departing from the coverage holes. Thus, there is a need to improve WWAN technology so that WWAN services can be seamlessly acquired by users experiencing poor or no coverage conditions.